With the gradual depletion of subterranean and shallow subsea hydrocarbon reservoirs, the search for additional petroleum reserves is being extended to deeper and deeper waters on the outer continential shelves of the world. As such deeper reservoirs are discovered, increasingly complex and sophisticated production systems have been developed. It is projected that in the near future, offshore exploration and production facilities will be required for probing depths of 10,000 feet or more. Since bottom founded compression structures are generally limited to water depths of no more than about 1,200-1,500 feet because of the sheer size of structure required, other, so-called compliant structures have been developed.
One type of compliant structure receiving considerable attention is a tension leg platform (TLP). A TLP comprises a semisubmersible-type floating platform anchored to the sea bed through vertical members or mooring lines called tension legs. The tension legs are maintained in tension at all times by insuring that the buoyancy of the TLP exceeds its operating weight under all environmental conditions. A TLP is compliantly restrained in the lateral directions allowing sway, surge and yaw while vertical plane movements such as heave, pitch and roll are stiffly restrained by the tension legs.
In the first commercially installed TLP built for the Hutton Field in the U.K. North Sea, installed in 485 feet of water, separate piled anchor templates were provided for anchoring the tensioned mooring elements extending from each of the four corner columns of the floating tension leg platform. A separate well template was also provided on the sea bed.
It has also been proposed to utilize a gravity base well template for the anchoring of a TLP and to serve as a guide template for wells located below the TLP. A gravity base well template is a massive structure which is held in place on the sea floor by gravity, rather than by anchored pilings.
One feature of a gravity base subsea well template which must be accommodated, however, is the tendency of such a massive gravity structure to gradually settle downward in the ocean floor throughout the operating life of the gravity base well template.